EP0779988B1 - Process and device for measuring an alternating electric current with temperature compensation - Google Patents

Abstract

PCT No. PCT/DE95/01138 Sec. 371 Date Mar. 7, 1997 Sec. 102(e) Date Mar. 7, 1997 PCT Filed Aug. 25, 1995 PCT Pub. No. WO96/07922 PCT Pub. Date Mar. 14, 1996Linearly polarized measuring light is broken down into two differently polarized component light signals in an analyzer after passing through a Faraday sensor device. The intensity of the corresponding electric intensity signals is normalized by dividing the respective alternating signal component by the respective direct signal component. From the two normalized intensity signals S1 and S2, a measuring signal is derived according to the formula: S=(2xS1xS2)/((S2-S1)+Kx(S1+S2)) where cos(2 theta +2 eta )=-2/(3K) and sin(2 theta -2 eta )=1 are valid for a correction factor K, an injection angle eta between the plane of polarization of the injected measuring light to a natural axis of the linear birefringence in the sensor device and a exit angle theta between this natural axis and a natural axis of the analyzer.

Description

The invention relates to a method and an apparatus for Measuring an electrical alternating current.

There are optical measuring methods and measuring devices for measuring of an electric current, in which the magneto-optical Faraday effect is used. Under the Faraday effect one understands the rotation of the plane of polarization of linearly polarized light depending on a magnetic field. The angle of rotation is proportional to the path integral over the magnetic field along the distance traveled by the light Way with the Verdet constants as proportionality constants. The Verdet constant depends on that Material in which the light runs and on the wavelength of light. For measuring an electrical current in one Conductor near the conductor is the Faraday effect pointing sensor device arranged from an optically transparent material and in general Glass is made and with one or more, a light path forming solid bodies or with an optical waveguide can be formed. Through the sensor device sent linearly polarized light. The electrical one Electricity generated magnetic field causes rotation of the Polarization plane of the light in the sensor device around an angle of rotation by an evaluation unit as a measure of the strength of the magnetic field and thus for the strength of the electrical current can be evaluated. In general surrounds the sensor device, the current conductor, so that the polarized light the conductor in a quasi closed Path runs around. In this case, the amount of the polarization rotation angle in good approximation directly proportional to Amplitude of the measuring current.

In an embodiment known from W0 91/01501 optical measuring device for measuring an electric current is the sensor device as part of an optical single-mode fiber trained the current conductor in the form of a Measuring winding surrounds. The polarized measuring light circulates around the Conductors therefore N times during a run, if N is the number is the turns of the measuring winding. With the so-called Transmission type, the measuring light only passes through the measuring winding once. On the other hand, in the reflection type, the other end is the Mirrored fiber, so that the measuring light after a first Run the measurement winding a second time in reverse Direction goes through. Because of the non-reciprocity of the Faraday effect is therefore the angle of rotation for the reflection type with the same measuring winding twice as large as the transmission type.

From EP-B-0 088 419 an optical measuring device for Measuring a current is known in which the sensor device is designed as a solid glass ring around the current conductor. Light from a light source becomes linear with a polarizer polarized and then coupled into the sensor device. The linearly polarized light passes through the sensor device once and then using a wollaston prism as polarizing beam splitter into two linearly polarized Partial light signals A and B with perpendicular to each other Divided polarization planes. Each of these two light signals A and B is via an associated optical transmission fiber transmitted to an associated light detector and in a corresponding electrical signal PA and PB converted. Out these two signals PA and PB is in a processing unit an intensity-standardized measurement signal M = (PA-PB) / (PA + PB) is formed. This measurement signal M is independent of intensity fluctuations the light source or attenuation in the optical Supply lines.

A problem with such optical measuring methods and measuring devices for current measurement there are interferences from additional linear birefringence in optical materials the sensor device and the optical transmission links Such additional linear birefringence can be achieved by mechanical Tensions caused, for example, by bending or Vibrations are caused, or by temperature changes caused. This caused by disturbances linear birefringence leads to an undesirable change the operating point and the sensitivity.

There are already several for compensating for temperature influences Temperature compensation method known.

In US 4,755,665 a temperature compensation method for a magneto-optical measuring device for measuring alternating currents suggested. In this process, the analog to the previously described measuring device known from EP-B-0 088 419 obtained electrical signals PA and PB respectively in a filter into their direct current components PA (DC) or PB (DC) and their AC components PA (AC) and PB (AC) disassembled. Out the AC component PA (AC) or PB (AC) and the DC component PA (DC) or PB (DC) is used for each signal PA and PB to compensate for different intensity fluctuations in the two transmission links for the Light signals A and B with the quotient QA = PA (AC) / PA (DC) or QB = PB (AC) / PB (DC) from its AC component PA (AC) or PB (AC) and its direct current component PA (DC) or PA (DC) are formed. Each of these two quotients QA and QB becomes one temporal mean MW (QA) and MW (QB) formed, and from these two averages MW (QA) and MW (QB) will eventually a quotient Q = MW (QA) / MW (QB) is formed. As part of a Iteration process is done by comparison with in a table of values (Look-up table) stored, calibrated values Correction factor K obtained for the quotient Q determined. The value Q * K corrected by this correction factor K is called Temperature compensated measured value for an electrical to be measured AC used. With this procedure the temperature sensitivity can be reduced to about 1/50.

Another temperature compensation method is known from EP-A-0 557 090 for an optical measuring device for measuring magnetic Alternating fields known that exploits the Faraday effect and therefore also for measuring electrical alternating currents suitable is. In this known method, this becomes linear polarized measuring light after passing through a sensor device in one analyzer in two different linearly polarized Split light signals A and B, and it will for intensity normalization for each of the two associated electrical signals PA and PB separately the quotient QA = PA (AC) / PA (DC) or QB = PB (AC) / PB (DC) from its associated AC component PA (AC) or PB (AC) and its associated DC component PA (DC) or PB (DC) formed. From the two Quotients QA and QB is now entered in a computing unit Measurement signal M = 1 / ((α / QA) - (β / QB)) formed with the real constants α and β, which satisfy the relation α + β = 1. This Measurement signal M is largely independent of temperature changes caused changes in the Verdet constant and the circular birefringence described in the sensor device. Via compensation of the temperature-induced linear birefringence says nothing. The constants α and β are determined experimentally.

The invention is based on the object of a method and a device for measuring an electrical alternating current with the help of a sensor device showing the Faraday effect specify where influences from temperature-induced linear birefringence and intensity fluctuations largely compensated for the measurement signal.

This object is achieved according to the invention with the features of claim 1 and claim 2. In the optical sensor device under the influence of a magnetic field generated by the electrical alternating current, linearly polarized measuring light is coupled in via coupling means. When passing through the sensor device, the polarization plane of the measuring light is changed as a function of the electrical alternating current. After passing through the sensor device, the measuring light is divided by an analyzer into two linearly polarized partial light signals with different polarization levels. These two partial light signals are then each converted into corresponding electrical intensity signals by optoelectric converters. Each of these two intensity signals is separately intensity-normalized by the quotient signal being formed from its alternating signal component and its DC signal component by standardizing means. As a result, fluctuations in the intensity of the optical coupling means and in the transmission links can be compensated for the two partial light signals. The two quotient signals as intensity-normalized signals I1 and I2 are now used by evaluation means to produce a measurement signal S for the electrical alternating current I in accordance with the regulation S = (2 * S1 * S2) / ((S2-S1) + K * (S1 + S2)) derived, where K is a real correction factor and this correction factor K, the coupling angle η of the polarization plane of the measuring light coupled into the sensor device to a natural axis of the linear birefringence in the sensor device and the so-called coupling angle  between this natural axis of the linear birefringence and a natural axis of the analyzer meet the following two conditions: cos (2 + 2η) = - 2 / (3K) sin (2 - 2η) = 1. Advantageous embodiments of the measuring method and the measuring device according to the invention result from the respective dependent claims.

FIG. 1

ein prinzipieller Aufbau einer Vorrichtung zum Messen eines elektrischen Wechselstromes,

FIG. 2

eine Ausführungsform von Normierungsmitteln für eine solche Meßvorrichtung,

FIG. 3

eine Ausführungsform von Auswertemitteln zur Temperaturkompensation für eine solche Meßvorrichtung und

FIG. 4

ein Einkoppelwinkel η und ein Auskoppelwinkel  in einem Diagramm

schematisch veranschaulicht sind. Einander entsprechende Teile sind mit denselben Bezugszeichen versehen.To further explain the invention, reference is made to the drawing in which

FIG. 1

a basic structure of a device for measuring an electrical alternating current,

FIG. 2nd

one embodiment of standardization means for such a measuring device,

FIG. 3rd

an embodiment of evaluation means for temperature compensation for such a measuring device and

FIG. 4th

a coupling angle η and a coupling angle  in a diagram

are illustrated schematically. Corresponding parts are provided with the same reference numerals.

FIG. 1 shows an embodiment of the measuring device for measuring an electrical alternating current I in a conductor 2 in a basic structure. The current conductor 2 is one Associated sensor device 3 which, under the influence of the magnetic alternating current I generated the magnetic field Polarization of radiation radiated into the sensor device 3 linearly polarized measuring light depending on the electrical AC current I changes. The sensor device 3 exists to do this at least partially from at least one magneto-optical Faraday effect material. In the Sensor device 3 is coupled in linearly polarized measuring light L. To generate this linearly polarized measuring light L can be a simple light source and associated, not shown polarizing means or even one polarizing light source 4, for example a laser diode, be provided. The sensor device 3 and the light source 4 are preferably a polarization-maintaining Light guide 34, for example a single-mode optical fiber such as HiBi (high birefringence) fiber or polarization neutral LoBi (low birefringence) fiber, optically connected. However, a multimode light guide can also be used, for example an optical fiber used in telecommunications will. Especially in the latter case, the light the light source 4 directly before being coupled into the sensor device 3 sent through an additional polarizer and linearly polarized. The linearly polarized measuring light L passes through the sensor device 3 at least once and experiences one of the electrical alternating current I in the conductor 2 dependent change in its polarization. The decoupled Measuring light L has one due to the Faraday effect Measuring angle α, not shown, rotated plane of polarization on. The measuring angle α is dependent on an alternating current I in the current conductor 2.

The sensor device 3 can be provided with a light guide, preferably an optical fiber, which is the current conductor 2 in a measuring winding with at least one measuring winding surrounds. The light guide of the sensor device 3 is with the Light guide 34 for supplying the measuring light L then preferably connected by a splice. Can as sensor device 3 but also one or more solid bodies made of Faraday materials be provided, which is preferably closed Form light path around the current conductor 2, for example a Glass ring. However, the sensor device 3 must have the current conductor 2 not in a closed light path, but can also arranged in close proximity to the current conductor 2 be.

After passing through the sensor device 3, the measuring light L fed to an analyzer 7 and in the analyzer 7 in split two linearly polarized light signals L1 and L2, whose polarization planes are different from each other. Preferably are the polarization planes of the two partial light signals L1 and L2 directed perpendicular to each other (orthogonal Disassembly). A polarizing one can be used as the analyzer 7 Beam splitter, for example a Wollaston prism, or also two at an appropriate angle and preferably 90 ° crossed polarization filters and a simple beam splitter be provided with a partially transparent mirror. The sensor device 3 and the analyzer 7 can have a free jet arrangement or via a polarization-maintaining Optical fiber, preferably a single-mode optical fiber such as a HiBi (high birefringence) fiber or one polarization-neutral LoBi (low birefringence) fiber, optical be connected.

The two partial light signals L1 and L2 are then each a photoelectric converter 12 and 22 respectively. The Transmission of the two partial light signals L1 and L2 from the analyzer 7 to the associated transducer 12 or 22 can over a free jet arrangement or preferably over each a light guide 11 or 21 take place. In the converters 12 and 22, the two partial light signals L1 and L2 are each in converted an electrical intensity signal I1 or I2, which is a measure of the intensity of the associated partial light signal L1 or L2 is.

The electrical intensity signals I1 and I2 now become standardization means 20 fed. At one exit each Standardizing means 20 are the intensity-standardized signals S1 and S2 on. As intensity-standardized signals S1 and S2 the quotient of the alternating signal component and the DC signal component of the associated intensity signal I1 or I2 formed. The intensity norms thus formed Signals S1 and S2 are compensated for fluctuations in intensity, i.e. Fluctuations in the light intensities in particular Microbending losses in the light guides and due to vibrations or other mechanical influences and through Fluctuations in the intensity of the light source 4 are practical eliminated. Also different intensity changes in the two optical transmission paths for the partial light signals L1 and L2 are through this intensity normalization compensated. Therefore, as transmission links for the two light part signals L1 and L2 also each have a multimode fiber be provided.

However, changes in temperature are a problem due to temperature induced linear birefringence in the optical materials of the optical measuring device, in particular the sensor device 3, and the associated Shift of the working point and in particular the change the measuring sensitivity of the measuring device. This temperature-induced A change in the sensitivity is indicated by an im following described temperature compensation method with With the help of evaluation means 40 largely compensated.

For this purpose, the two intensity-standardized signals S1 and S2 each fed to an input of the evaluation means 40. The Evaluation means 40 form the two intensity-standardized Signals S1 and S2 an output signal as a measurement signal S, the at least approximately satisfies the aforementioned equation (1), essentially the quotient of the double product 2 * S1 * S2 of the two intensity-standardized signals S1 and S2 and the sum of the difference S2-S1 and that with a correction factor K multiplied sum S1 + S2 of the two intensity normalized Corresponds to signals S1 and S2.

The correction factor K is a real number that depends from one in FIG. 4 coupling angle η shown Polarization plane P of the one coupled into the sensor device 3 linearly polarized measuring light L to a natural axis EF the linear birefringence of the sensor device 3 on the one hand and from one also in FIG. 4 decoupling angle shown  between this eigen axis EF and an eigen axis EA of the analyzer 7 is set so that at least approximately meets the aforementioned conditions (2a) and (2b) are. From equation (2a) it follows in particular that for the Correction factor K ≤ -2/3 or K ≥ 2/3 applies. A self axis of a birefringent material is due to the state of polarization of the measuring light L defines the material leaves unchanged. In FIG. 4 defines the drawing level a plane that is perpendicular to the direction of propagation of the Measuring light L is directed.

Deviations from the conditions (2a) and (2b) Precisely fulfilling angle values are especially great linear and / or circular birefringence in the sensor device 3 possible and can be up to about 5 °.

Calculations have shown that with such a coupling angle η and coupling angle  chosen as a function of the correction factor K, the measurement signal S has a particularly simple dependency on the Faraday rotation angle or measurement angle α. The relationship then applies to a good approximation S = sin (2α)

This measurement signal S corresponds to the theoretical measurement signal for the measurement angle α without birefringence effects. This will the evaluation process is particularly simple. At given Coupling and decoupling angles η and  is the correction factor K just set so that the measurement signal S of the measuring device the theoretical measurement signal when none Birefringence corresponds. With this correction factor K is the measuring device then to minimal temperature dependence adjusted.

If the measuring light L before coupling into the sensor device 3 can be linearly polarized with a polarizer as the coupling angle η simply the angle between the natural axis this polarizer 5 and a natural axis of the linear Birefringence can be selected in the sensor device 3.

The measurement signal S is both against fluctuations in intensity the light source and in the optical transmission links as also with regard to its working point and its sensitivity largely stable at variable temperatures. The arithmetic derivation of the measurement signal S according to the Equation (1) is preferably created using appropriate analog components for the arithmetic to be performed Operations performed. In this embodiment, one is Temperature compensation possible in real time. The measurement signal S can also be digital using a digital signal processor or a microprocessor and / or calculated with Can be determined using a stored value table.

In addition to the one shown in FIG. 1 shown embodiment of the transmission type, in which the measuring light L is the sensor device 3 only runs in one direction is also an embodiment of the reflection type possible, in which the measuring light L after a first pass back into the sensor device 3 and the sensor device 3 a second time in the opposite direction. The linearly polarized Measuring light L of the light source 4 is then preferably over a beam splitter at an optical connection in the sensor device 3 coupled in and after the first pass reflected by a mirror, passes through the sensor device 3 a second time, it is coupled out again at the named connection and fed to the analyzer 7 via the beam splitter.

FIG. 2 shows an advantageous embodiment of the standardization means 20. There is a first, with the optoelectric Converter 12 electrically connected filter 13 for disassembling the first intensity signal I1 into an alternating signal component A1 and a DC signal component D1 and a second one with which second optoelectric converter 22 electrically connected Filter 23 for breaking down the second intensity signal I2 into an AC signal component A2 and a DC signal component D2 intended. The cut-off frequencies of both filters 13 and 23 are chosen so high that the alternating signal components A1 and A2 the intensity signals essentially all information contained about the alternating current I to be measured. Especially the crossover frequency is lower than the base frequency of the alternating current I choose. In the illustrated embodiment Both filters 13 and 23 each contain a low-pass filter 14 or 24 and a subtractor (SUB) 15 or 25. The intensity signal I1 or I2 is assigned to an input of the Low pass filter 14 and 24 created. At an exit of the Low pass filter 14 or 24 is then the DC signal component D1 and D2 of the intensity signal I1 and I2 on the frequency components corresponds to the intensity signal I1 or I2, the below a predetermined cut-off frequency of the low-pass filter 14 or 24 are. The respective alternating signal component Al or A2 is now simply subtracted from the DC signal component D1 or D2 from the intensity signal I1 or I2 with With the help of a subtractor 15 and 25 respectively. To are at two inputs of the associated subtractor 15 or 25 the intensity signal I1 or I2 and the associated one DC signal component D1 or D2 applied.

Instead of the in FIG. 2 shown embodiments of the filter 13 and 23 can of course also high-pass filters and low-pass filters to filter out the alternating signal components A1 and A2 or the DC signal components D1 and D2 can be provided or also a high-pass filter to filter out the AC signal component Al or A2 and a subtractor for deriving of the DC signal component D1 = I1-A1 or D2 = I2-A2.

The AC signal component Al and the DC signal component D1 of the first intensity signal I1 are now one input each a divider 16 supplied. At an exit of the divider 16 is then the first intensity-standardized signal S1 = A1 / D1 as the quotient of AC signal component A1 and DC signal component D1 of the first intensity signal I1. As well become the AC signal component A2 and the DC signal component D2 of the second intensity signal I2 a second divider 26 supplied, the intensity-standardized signal S2 = A2 / D2 forms for the second intensity signal I2. The two intensity-standardized Signals S1 and S2 can then be sent to corresponding Outputs of the standardization means 20 are tapped.

The FIG. 3 shows an embodiment of the evaluation means 40 with analog hardware. A multiplier (MULT) 41, a first adder (ADD) 42, a subtractor (SUB) 43, two amplifiers 44 and 45, a second adder (ADD) 46 and a divider (DIV) 47 are provided. The two intensity-normalized signals S1 and S2 of the normalization means 20 are each fed to two inputs of the multiplier 41, the first adder 42 and the subtractor 43. The product S1 * S2 of the two signals S1 and S2 at the output of the multiplier 41 is fed to an input of the traitor 44. The amplifier 44 is set to a gain factor 2, so that the double product 2 * S1 * S2 of the two signals S1 and S2 is present at its output. The sum S1 + S2 of the two signals S1 and S2 at the output of the first adder 42 is applied to an input of the further amplifier 45 and amplified by the correction factor K. For this purpose, the amplification factor of the amplifier 45 is to be set to the predetermined correction factor K. The product K * (S1 + S2) at the output of the amplifier 45 is fed to an input of the second adder 46. At the other input of this adder 46, the output signal S2-S1 of the subtractor 43 is applied, which corresponds to the difference between the two signals S1 and S2. The adder 46 forms the output signal K * (S1 + S2) + (S2-S1) as the sum of its two input signals. Finally, the output signal 2 * S1 * S2 of the amplifier 44 is fed to the first input of the divider 47 and the output signal of the second adder 46 to the second input of the divider 47. The divider 47 calculates the measurement signal from these two input signals S = (2 * S1 * S2) / (K * (S1 + S2) + (S2-S1)) according to equation (1). This embodiment with analog components according to FIG. 3 has the advantage that the calculation of the measurement signal S is carried out particularly quickly. In particular in combination with standardization means 20 according to FIG. 2, temperature compensation is possible in real time.

The measuring method and the measuring device according to the invention can of course also be used to measure magnetic Alternating fields can be used by the sensor device 3 is arranged in the alternating magnetic field.

Claims (7)

1. Method for measuring an electric alternating current (I) by using the Faraday effect, having the following features:

   a) linearly polarized measuring light (L) is coupled into a sensor equipment (3) which is under the influence of a magnetic field generated by the electric alternating current (I), and the plane of polarization of the measuring light (L) is rotated while passing through the sensor equipment (3), as a function of the electric alternating current (I);

   b) the measuring light (L), after passing at least once through the sensor equipment (3), is split by an analyser (7) into two linearly polarized light partial signals (L1, L2) having different planes of polarization;

   c) the two light partial signals (L1, L2) are in each case converted into an electric intensity signal (I1, I2);

   d) for each of these two electric intensity signals (I1, I2) an intensity-normalized signal S1 and S2 is formed, which corresponds to the quotient of an AC signal component and a DC signal component of the associated intensity signal (I1, I2);

   e) a measured signal S for the electric alternating current I is now derived from the two intensity-normalized signals S1 and S2 in accordance with the rule S = (2*S1*S2)/((S2-S1) + K*(S1+S2)) where the real correction factor K, an input coupling angle η of the plane of polarization of the measuring light (L) coupled into the sensor equipment (3) to an intrinsic axis of the linear birefringence in the sensor equipment (3), and an output coupling angle  between this intrinsic axis of the linear birefringence and an intrinsic axis of the analyser (7) fulfil the following two conditions: cos(2 + 2η) = - 2/(3K) sin(2 - 2η) = 1.
2. Device for measuring an electric alternating current (I) having

   a) input coupling means (4, 34) for coupling linearly polarized measuring light (L) into a sensor equipment (3) which is under the influence of a magnetic field generated by the alternating current (I), and rotates the plane of polarization of the measuring light (L) as a function of the electric alternating current (I),

   b) an analyser (7) for splitting the measuring light (L), after passing at least once through the sensor equipment (3), into two linearly polarized light partial signals (L1, L2) having different planes of polarization,

   c) optoelectric converters (12, 22) for converting the two light partial signals (L1, L2) into one electric intensity signal (I1, I2) in each case,

   d) normalizing means (13, 23) which, for both intensity signals (I1, I2) in each case form an intensitynormalized signal S1 and S2 which corresponds to the quotient of an AC signal component and a DC signal component of the associated intensity signal (I1, I2);

   e) evaluation means (20) for deriving a measured signal S for the electric alternating current (I) from the two intensity-normalized signals S1 and S2 in accordance with the rule S = (2*S1*S2)/((S2-S1) + K*(S1+S2)), where K is a real correction factor and this correction factor K, an input coupling angle η of the plane of polarization of the measuring light (L) coupled into the sensor equipment (3) to an intrinsic axis of the linear birefringence in the sensor equipment (3), and an output coupling angle  between this intrinsic axis of the linear birefringence and an intrinsic axis of the analyser (7) fulfil the following two conditions: cos(2 + 2η) = - 2/(3K) sin(2 - 2η) = 1.
3. Device according to Claim 2, in which a Wollaston prism is provided as analyser (7).
4. Device according to Claim 2 or Claim 3, in which the analyser (7) is optically connected to the optoelectric converters (12, 22) via in each case an optical waveguide (11, 21) for each light partial signal (L1, L2).
5. Device according to Claim 4, in which the optical waveguides (11, 21) are multimode fibres.
6. Device according to one of Claims 2 to 5, in which the sensor equipment (3) and the analyser (7) are optically connected to each other, for transmitting the measuring light (L) coupled out of the sensor equipment (3), via a polarization-maintaining optical fibre.
7. Device according to one of Claims 2 to 6, in which a glass ring is provided as sensor equipment (3).

Images